An exosome is a granular vesicle existing in a body fluid in a living body. It is known that a wide variety of membrane proteins exist on an exosomal surface, as in the case with a general cell surface. On the other hand, it has also become known that microRNA (miRNA) is contained inside an exosome, in addition to various proteins such as cytokines. In addition, it has been also reported that exosomes are secreted from various kinds of cells, for example, cells of the immune system and various cancer cells, and the function as an intermediary in intercellular communication in a living body to be associated with physiological phenomena and association with a disease such as cancer have been remarked.
For example, it has been reported in Non-Patent Publication 1 that an exosome in the circulating blood of a patient with ovarian cancer are isolated using an antibody against EpCAM, which is a tumor marker, and association between an expression level of miRNA derived from an exosome and patients with ovarian cancer is found. Accordingly, if a quantitative change in an exosome associated with a disease such as cancer can be easily grasped, its application to a diagnostic drug can be expected.
In addition, CD9, CD63 and CD81 are four-transmembrane membrane proteins belonging to tetraspanin family, and expressed on many exosomes. For example, it has been reported in Non-Patent Document 2 that an exosome in plasma of melanoma patients are detected and quantified with an antibody against CD63 and an antibody against a tumor-associated marker Caveolin-1, and that the exosomes increased as compared to those of normal individuals. In Patent Document 1, signals ascribed to exosomes in cancer patients are quantified and analyzed by subjecting centrifuged plasma samples to a reaction in combination with anti-CD63 antibodies, antibodies against various membrane proteins, and the like.